The phenomenon of negative magnetization and its implications

Abstract

In this article, we present an in-depth review of the phenomenon of negative magnetization (or magnetization sign reversal) with an up-to-date literature. We have described numerous experimental examples of the phenomenon, involving a variety of magnetically ordered systems, where it does not arise due to diamagnetism. The present review discusses physics principles for the sign reversal of magnetization under the following mechanisms: (a) negative exchange coupling among ferromagnetic sublattices, (b) negative exchange coupling among canted antiferromagnetic sublattices, (c) negative exchange coupling among ferromagnetic/canted-antiferromagnetic and paramagnetic sublattices, (d) imbalance of spin and orbital moments, and (e) interfacial exchange coupling between ferromagnetic and antiferromagnetic phases. We have put forward the roles of crystal structure, crystallite type (single crystal, bulk polycrystalline, thin film, and nanoparticle), lattice defect, electronic or chemical phase separation, magnetic anisotropy, and magnetic exchange interactions in the magnetization reversal. This review validates the mean field theory, given by L. Néel (1948), for an explanation of the negative magnetization under the category (a). We also bring out the necessity of further theoretical work to account for the other categories, (b)–(e). The present review also describes the importance of various magnetization measurement protocols for the occurrence of the magnetization reversal. Finally, we have pointed out the tunability aspect of the phenomenon. We conclude that the practical utilization of this phenomenon in magnetic memory, and magnetocaloric and spin resolving devices might be realized by choosing appropriate and well characterized materials whose compensation temperature can be tuned to room temperature.